Add deployment strategy to vtgate

[frouioui]

Description

This PR fixes #533 by allowing end-users to define a rolling update strategy (https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#strategy) for their vtgate deployments.

The upgrade test has been modified to ensure the strategy is correctly applied to the vtgate deployment.

This Pull Request must be merged/reviewed alongside with vitessio/vitess#18063.

[Copilot]

Pull Request Overview

This PR introduces a deployment strategy field to the vtgate CRD so that users can customize the vtgate deployment in Kubernetes similarly to standard Deployment objects. Key changes include:

• Adding a new "strategy" field in the vtgate CRD and updating related deep copy functions.
• Modifying the operator code to propagate the new deployment strategy field.
• Adjusting the CRDs, YAML test files, and documentation to reflect the new deployment strategy support.

Reviewed Changes

Copilot reviewed 11 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/endtoend/operator/operator-latest.yaml	Adds the deployment strategy field to the vtgate operator spec.
test/endtoend/operator/cluster_upgrade.yaml	Configures the deployment strategy in the upgrade test workload.
pkg/operator/vtgate/deployment.go	Sets the new Strategy field when updating the deployment.
pkg/controller/vitesscell/reconcile_vtgate.go	Propagates the new Strategy field from the CRD to the deployment.
pkg/apis/planetscale/v2/zz_generated.deepcopy.go	Adds deep copy handling for the new Strategy field.
pkg/apis/planetscale/v2/vitesscell_types.go	Updates the CRD spec to include the Strategy field.
docs/api.md	Documents the new strategy field in the API reference.
deploy/role_binding.yaml	Renames a role binding from vitess-operator-subcontroller to vitess-operator-backup-storage.
deploy/role.yaml	Renames a role from vitess-operator-subcontroller to vitess-operator-backup-storage.
deploy/crds/planetscale.com_vitessclusters.yaml	Adds the deployment strategy schema to the CRD.
deploy/crds/planetscale.com_vitesscells.yaml	Adds the deployment strategy schema to the CRD.

Files not reviewed (3)

• docs/api/index.html: Language not supported
• go.mod: Language not supported
• test/endtoend/upgrade_test.sh: Language not supported

Comments suppressed due to low confidence (2)

deploy/role_binding.yaml:4

• [nitpick] The role binding name change to 'vitess-operator-backup-storage' may be unexpected since the PR title focuses on adding deployment strategy to vtgate. Please confirm that this renaming aligns with the intended responsibilities and update documentation if necessary.

name: vitess-operator-backup-storage

deploy/role.yaml:4

• [nitpick] The role name change to 'vitess-operator-backup-storage' deviates from the expected naming convention for vtgate. Verify that this change is intentional and update related references or documentation accordingly.

name: vitess-operator-backup-storage

[frouioui]

This is from a previous bad commit, we had inconsistencies, sometimes it was -subcontroller and sometimes -backup-storage.

Modifying everything back to -backup-storage.